One form of process for producing hollow bodies or articles from oriented thermoplastic material comprises taking a hollow preform which comprises a cylindrical wall portion and a bottom portion, heating the cylindrical wall portion of the preform and then subjecting it to stretching at a starting temperature which is below the vitreous transition temperature, with the wall thickness of the cylindrical portion of the preform being reduced at the same time, the stretching effect being produced by a drawing ring that has an internal support shoulder for engaging the cylindrical wall portion of the preform thereagainst. In order to support the cylindrical wall portion of the preform which is to be subjected to stretching, a mandrel is inserted into the preform, the outside diameter of the mandrel substantially corresponding to the inside diameter of the cylindrical portion which is to be stretched.
Such a process is used, preferably when employing preforms of linear polyesters, for example polyethyleneteraphthalate (PETP) or polybutyleneteraphthalate (PBTP), for example when there is a need to produce containers for containing materials under a high pressure such as for example drinks containing carbon dioxide gas. The operating procedure involved in such a process is generally such that the heated preform which is of a generally cup-like configuration and which may be produced for example by injection molding is stretched over the above-mentioned mandrel, the length of which corresponds at least to that of the preform when stretched while, as mentioned, its outside diameter substantially corresponds to the inside diameter of the preform which is substantially the same as that of the article when in the stretched condition. The stretching operation generally involves a procedure in which, starting from the bottom end of the preform, the drawing ring is moved along the cylindrical wall portion thereof, towards its free end. During that movement of the drawing ring, the wall portion along which the ring moves is stretched for example to three to four times its initial length while at the same time the wall thickness thereof is also substantially reduced. The mandrel employed within the preform performs inter alia the function of supporting the wall portion to be stretched in that way in order thereby to prevent a reduction in the diameter thereof during the stretching operation.
In order to achieve a high level of output in that procedure, it is desirable for the stretching operation to be carried out at the highest speed that can be reasonably attained in order to minimise the amount of time required for that purpose. On the other hand however it is not possible to increase the rate at which stretching is effected, to just any random value, as that involves a risk of adversely affecting the quality of the end product. An aspect which is of importance in that connection is that the temperature of the preform and the allowable speed of stretching are interrelated insofar as the stretching speed can be increased with increasing temperature. The use of higher speeds of stretching requires the preform to be heated beforehand, but the temperature thereof should be below the vitreous transition temperature.
In order to achieve the high level of output which is required for economical manufacture, the individual steps in the process must be carefully matched to each other, while between the individual working stations involved in carrying out the process, conveyor means must be provided to transport the preforms from one station to another and possibly also within the stations, in the required time sequence. Thus, disposed upstream of the working station at which the stretching operation is carried out is a temperature control or heating station at which the preform is heated to a temperature which is as high as possible while however still being below the vitreous transition temperature. When using polyethyleneteraphthalate (PETP) the temperatures involved may be for example in the region of between 65.degree. C. and 75.degree. C. Other temperature ranges may be used when dealing with different plastic materials. In all cases however, although the temperatures are below the vitreous transition temperature of the plastic material used, the preform when preheated in that way, prior to the stretching operation, has a certain degree of plastic deformability. When the heated preform is being handled, for example when it is being transported from the temperature control or heating station into the following working station at which the stretching operation is performed on the heated preform, the fact that the heated preform is plastically deformable means that it may suffer from undesired deformation, particularly at its end region which is remote from the bottom portion thereof, and that may result in the cylindrical wall portion becoming out-of-round so that in the next following working station it is not possible, or not easily possible, for the mandrel to be inserted properly into the heated preform. In that connection account should be taken of the fact that the free end of the mandrel, which is the leading end to be inserted into the preform, may at most be only slightly conically tapered in order to facilitate insertion thereof, as in the effective position of the mandrel, that is to say during the stretching operation using the drawing ring, the free end of the mandrel must bear snugly against the transitional portion of the preform, between the bottom portion thereof and the cylindrical wall portion, in order to provide satisfactory support in that area as it is in or close to that area that the drawing or stretching effect begins. In order for the stretching operation to be satisfactorily performed, involving forces of the order of several thousands of kN, it is essential that the drawing ring comes into proper engagement with the cylindrical wall portion of the preform in the initial phase of the stretching operation. For that purpose, it is essential for the preform to be correctly supported precisely at the above-mentioned transitional region between the bottom portion and the wall portion of the preform. That support effect could not be achieved in the required manner if the mandrel were to be tapered at its leading end to facilitate insertion into the preform.